Date Published: December 19, 2013
Publisher: Public Library of Science
Author(s): Aydan Salman-Dilgimen, Pierre-Olivier Hardy, Justin D. Radolf, Melissa J. Caimano, George Chaconas, D. Scott Samuels.
The Lyme disease spirochete Borrelia burgdorferi must differentially express genes and proteins in order to survive in and transit between its tick vector and vertebrate reservoir. The putative DEAH-box RNA helicase, HrpA, has been recently identified as an addition to the spirochete’s global regulatory machinery; using proteomic methods, we demonstrated that HrpA modulates the expression of at least 180 proteins. Although most bacteria encode an HrpA helicase, RNA helicase activity has never been demonstrated for HrpAs and the literature contains little information on the contribution of this protein to bacterial physiology or pathogenicity. In this work, we report that B. burgdorferi HrpA has RNA-stimulated ATPase activity and RNA helicase activity and that this enzyme is essential for both mammalian infectivity by syringe inoculation and tick transmission. Reduced infectivity of strains carrying mutations in the ATPase and RNA binding motif mutants suggests that full virulence expression requires both ATPase and coupled helicase activity. Microarray profiling revealed changes in RNA levels of two-fold, or less in an hrpA mutant versus wild-type, suggesting that the enzyme functions largely or exclusively at the post-transcriptional level. In this regard, northern blot analysis of selected gene products highly regulated by HrpA (bb0603 [p66], bba74, bb0241 [glpK], bb0242 and bb0243 [glpA]) suggests a role for HrpA in the processing and translation of transcripts. In addition to being the first demonstration of RNA helicase activity for a bacterial HrpA, our data indicate that the post-transcriptional regulatory functions of this enzyme are essential for maintenance of the Lyme disease spirochete’s enzootic cycle.
Lyme borreliosis is the most prevalent vector-transmitted disease in the northern hemisphere and has a significant impact on human health (see , ). The disease is caused by the spirochete Borrelia burgdorferi and related species. B. burgdorferi is maintained in nature by a complex enzootic cycle that involves ticks as vectors and vertebrate animals as reservoir hosts. Survival in the very disparate arthropod and animal environments demands changes in the expression of numerous genes in a precise manner , , . The primary global regulators for these differentially-expressed genes are the alternative sigma factors RpoN and RpoS, which substitute for RpoD (σ70) in the RNA polymerase holoenzyme to effect transcription in response to environmental signals perceived during tick transmission and the mammalian phase of the enzootic cycle , , . Other players in the RpoN-RpoS pathway are the response regulatory protein Rrp2 , , ,  and the Fur/PerR ortholog, BosR , .